The present invention relates to magnetic field gradient sensor devices, and more particularly, to a magnetic field gradient sensor device incorporating an optically powered resonant integrated microstructure (O-RIM).
In a typical O-RIMS (optically powered-resonant integrated microstructure) device, a microbeam having a resonant frequency is held by supports and vacuum encapsulated by a polysilicon shell. The microbeam and the shell are supported by a silicon substrate, all of which together form a micromachined integrated silicon device. A typical O-RIMS device is further provided with an optical fiber which is positioned in proximity to the resonant microbeam.
Light generated by a light emitting diode (LED), laser or other light source, arrives at the O-RIMS device via the optical fiber, passes through the shell, partially through the microbeam, and on to a photodiode situated beneath the resonant microbeam. The shell is partially reflective and partially transparent to the light delivered by the optical fiber. The shell, the microbeam and the substrate create a Fabry-Perot cavity, such that light waves reflected from these surfaces constructively and destructively interfere with one another as they re-enter the optical fiber, creating an optical signal whose intensity changes as the microbeam moves up and down.
The microbeam is excited to resonance by the arrival of the light through the optical fiber striking the photodiode causing charge to build up there, creating an electrostatic attraction to the microbeam. The electrostatic attraction causes the microbeam to flex, and as the microbeam approaches its maximum flexure, its potential energy builds to a point where its restoring force overcomes the electrostatic attraction. The microbeam then springs toward a neutral or resting position, where the electrostatic attraction builds again, flexing the microbeam again, and exciting resonance in the microbeam.
The addition of a suitable ferromagnetic or magnetically permeable element and fulcrum to the typical O-RIMS structure, transforms the O-RIMS into a device that is sensitive to the presence of a magnetic field gradient Therefore, an O-RIMS incorporating a ferromagnetic or magnetic element according to the design and function of the present invention, becomes a highly accurate and reliable magnetic field gradient sensor.
Thus, the magnetic field gradient sensor of the present invention is optically driven and optically read, is small in size, and no electrical power supplies or wires are required either on the device or external to the device. Therefore, since no electrical connections are required, device packaging is greatly simplified and the operative component of the device can comprise the O-RIMS structure on an appropriately designed die bonded directly to the tip of a light transporter, such as an optical fiber or an optical waveguide. Hence, the complete sensor can have a diameter no bigger than the tip of the light transporter used.
Moreover, the present invention is fast, such that depending on design parameters of the microbeam, shell, fulcrum, and ferromagnetic or magnetically permeable element, magnetic field gradient measurements could be taken in a millisecond or less. The magnetic field gradient sensor of the present invention requires very little optical power, typically an optical signal of only several microwatts or less should be sufficient to drive it.
By modulating a high-frequency carrier (typically in the range of hundreds of kHz), this magnetic field gradient sensor is relatively immune to 1/f noise. In addition, due to the fact that the photodiode is physically located close to the microbeam, and any distant communication between the sensor electronics and the photodiode is via the light transporter, and since a light transporter intended for use with the present invention eliminates signal-to-noise ratio problems that arise with transmission of electrical signals through long metallic conductors, the present invention offers an even greater advantage in the signal-to-noise performance over long distances.
In accordance with one aspect of the present invention, a device for sensing a magnetic field gradient comprises a shell having an outer surface and an inner surface, a beam affixed to the inner surface of the shell, a magnetically permeable element in mechanical connection to the outer surface of the shell, a photodiode in proximity to the beam, and a light transporter having an end proximate to the outer surface of the shell.
In accordance with another aspect of the present invention, a method for sensing a magnetic field gradient using a magnetically permeable element mechanically coupled to a resonant beam comprises directing a first light wave to the beam, exciting the beam to a resonant frequency in response to the first light wave, and transmitting a second light wave having a property corresponding to the resonant frequency of the beam away from the beam.
In accordance with yet another aspect of the present invention, an optically-powered integrated microstructure magnetic field gradient sensor comprises a substrate, a microbeam, a photodiode, a cantileveredly supported magnetically permeable element, and an optical fiber. The substrate supports a polysilicon shell having an outer surface and an inner surface, the inner surface defining an evacuated cavity enclosing an area of the substrate, and the outer surface of the shell defines an area surrounded by supports extending from a plane of the substrate. The microbeam is affixed to the inner surface of the shell within the evacuated cavity by posts. The photodiode is integrated into the substrate at a surface location beneath the microbeam. The cantileveredly supported magnetically permeable element is suspended in mechanical connection to the outer surface of the shell within the area surrounded by the supports. The optical fiber has a distal end and a proximate end, and the proximate end is disposed at the outer surface of the shell within the area surrounded by the supports.